Rocky Mountain spotted fever (RMSF) is a bacterial infection spread by a bite from an infected tick. It causes vomiting, a sudden high fever around 102 or 103°F, headache, abdominal pain, rash, and muscle aches.

RMSF is considered the most serious tick-borne illness in the United States. Though the infection can be treated successfully with antibiotics, it can cause serious damage to internal organs, or even death if it isn’t treated right away. You can reduce your risk by avoiding tick bites or promptly removing a tick that has bitten you.

The symptoms of Rocky Mountain spotted fever typically begin between 2 and 14 days after getting a tick bite. Symptoms come on suddenly and usually include:

high fever, which may persist for 2 to 3 weeks

chills

muscle aches

headache

nausea

vomiting

fatigue

poor appetite

abdominal pain

RMSF also causes a rash with small red spots on the wrists, palms, ankles, and soles of the feet. This rash begins 2 to 5 days after the fever and eventually spreads inward towards the torso. After the sixth day of infection, a second rash can develop. It tends to be purple-red, and is a sign that the disease has progressed and become more serious. The goal is to begin treatment before this rash appears.

RMSF can be difficult to diagnose, as the symptoms mimic other illnesses, such as the flu. Although a spotted rash is considered the classic symptom of RMSF, about 10 to 15 percent of people with RMSF don’t develop a rash at all. Only about half of people who develop RMSF remember having a tick bite. This makes diagnosing the infection even more difficult.

RMSF is transmitted, or spread, through the bite of a tick that’s infected with a bacterium known as Rickettsia rickettsii. The bacteria spread through your lymphatic system and multiply in your cells. Though RMSF is caused by bacteria, you can only be infected with the bacteria via a tick bite.

There are many different types of ticks. Types that may be vectors, or carriers, of RMSF include the:

American dog tick (Dermacentar variablis)

Rocky Mountain wood tick (Dermacentor andersoni)

brown dog tick (Rhipicephalus sanguineus)

Ticks are small arachnids that feed on blood. Once a tick has bitten you, it may draw blood slowly over several days. The longer a tick is attached to your skin, the greater the chance of an RMSF infection. Ticks are very small insects — some as small as the head of a pin — so you may never see a tick on your body after it bites you.

RMSF is not contagious and can't be spread from person to person. However, your household dog is also susceptible to RMSF. While you can't get RMSF from your dog, if an infected tick is on your dog's body, the tick can migrate to you while you’re holding your pet.

Treatment for Rocky Mountain spotted fever involves an oral antibiotic known as doxycycline. It’s the preferred drug for treating both children and adults. If you’re pregnant, your doctor may prescribe chloramphenicol instead.

The CDC recommends that you start taking the antibiotic as soon as the diagnosis is suspected, even before your doctor receives the laboratory results needed to definitively diagnose you. This is because delay in treating the infection can lead to significant complications. The goal is to begin treatment as soon as possible, ideally within the first five days of infection. Make sure you take the antibiotics exactly the way your doctor or pharmacist described.

If you don't begin receiving treatment within the first five days, you might require intravenous (IV) antibiotics in the hospital. If your disease is severe or you have complications, you may have to stay in the hospital for a longer period of time to receive fluids and be monitored.

RMSF is rare, but the number of cases per million people, known as incidence, has been increasing over the last 10 years. The current number of cases in the United States is now around six cases per million people per year.

How common is RMSF?

Around 2,000 cases of RMSF are reported to the Centers for Disease Control and Prevention (CDC) each year. People who live close to wooded or grassy areas and people who are in frequent contact with dogs have a higher risk of infection.

Where is RMSF most commonly found?

Rocky Mountain spotted fever got its name because it was first seen in the Rocky Mountains. However, RMSF is more frequently found in the southeastern part of the United States, as well as parts of:

Canada

Mexico

Central America

South America

In the United States, 5 states see over 60 percent of RMSF infections:

North Carolina

Oklahoma

Arkansas

Tennessee

Missouri

What time of year is RMSF most commonly reported?

The infection can occur at any time of the year, but is more common during the warm weather months, when ticks are more active and people tend to spend more time outside. Most cases of RMSF occur during May, June, July, and August.

What is the fatality rate of RMSF?

RMSF can be fatal. However, in the United States overall, less than 1 percent of people infected with RMSF will die from the infection. Most fatalities occur in the very old or very young, and in cases where treatment was delayed. According to the CDC, children under 10 years of age are 5 times more likely to die from RMSF than adults.

You can prevent RMSF by avoiding tick bites or by removing ticks from your body promptly. Take these precautions to prevent a tick bite:

To prevent bites

Avoid densely wooded areas.

Mow lawns, rake leaves, and trim trees in your yard to make it less attractive to ticks.

Tuck your pants into your socks and your shirt into your pants.

Wear sneakers or boots (not sandals).

Wear light colored clothing so you can easily spot ticks.

Apply insect repellant containing DEET. Permethrin is also effective, but should only be used on clothing, not directly on your skin.

Check your clothes and body for ticks every three hours.

Perform a thorough check of your body for ticks at the end of the day. Ticks prefer warm, moist areas, so be sure to check your armpits, scalp, and groin area.

Scrub your body in the shower at night.

If you do find a tick attached to your body, don't panic. Proper removal is important to decrease the likelihood of infection. Follow these steps to remove the tick:

To remove ticks

Using a pair of tweezers, grasp the tick as close to your body as possible. Do not squeeze or crush the tick during this process.

Pull the tweezers upward and away from the skin slowly until the tick detaches. This may take a few seconds and the tick will probably resist. Try not to jerk or twist.

After removing the tick, cleanse the bite area with soap and water and disinfect your tweezers with rubbing alcohol. Make sure to also wash your hands with soap.

Place the tick in a sealed bag or container. Rubbing alcohol will kill the tick.

If you feel ill or develop a rash or a fever after having a tick bite, see your doctor. Rocky Mountain spotted fever and other diseases transmitted by ticks can be dangerous if they’re not treated right away. If possible, take the tick, inside the container or plastic bag, with you to the doctor's office for testing and identification.

Medically reviewed by Judith Marcin, MD on April 13, 2017 — Written by Jacquelyn Cafasso